Integrated circuit probe card inspection system

ABSTRACT

A system for inspecting integrated circuit probe cards using a video camera positioned to view probe points on the cards from below. A precision movement stage is used to move the video camera into a known position for viewing the probe points. Analysis of the video image and the stage position are used to determine the relative positions of the probe points.

This is a division of application Ser. No. 80/072,206, filed Jun. 4,1993 now U.S. Pat. No. 5,657,394.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the testing of integratedcircuits, and more particularly, to a method and apparatus for testingprobe cards which are used to make electrical contact with integratedcircuit chips in wafer form.

BACKGROUND OF THE INVENTION

Integrated circuits are fabricated by a series of batch processes inwhich wafers, typically of silicon or other compatible material, areprocessed to produce a particular type of integrated circuit. Each wafercontains a plurality of integrated circuits or chips, typically of thesame kind. As is known, chemical, thermal, photolithographic andmechanical operations are typically involved in the fabrication of theintegrated circuit wafer. Because of variations across the wafer andacross each individual chip caused by process variables or physicalphenomena, however, not all chips on the wafer will meet the desiredspecifications for the chips. Some method of testing must be employed todetermine which chips on any given wafer meet the specifications.

Integrated circuit chips are typically fabricated with one or morelayers of metal interconnect on the surface of the chip which provideconnecting paths to form the desired circuit. The metal interconnectlayer or layers also provide a means to make connections to theintegrated circuit chip when the chip is separated from the wafer and isassembled into a package or carrier. Interconnect points, typicallycalled "bonding pads", are formed by the metal interconnect and arearrayed on the surface of the chip so as to allow small bonding wires orother connecting means to be connected from the integrated circuit chipto its carrier or package. These same bonding pads, and others designedspecifically for test purposes, are used to make electrical contact toeach individual chip for testing the electrical characteristics of thechips even while still joined together in wafer form.

The yield of good chips on a wafer is defined as the percentage of gooddies with respect to the total dies present on the wafer. Yield is thesingle most important cost factor in the production of integratedcircuit devices. Each process and test step may be considered apotential yield loss point. The testing of each die on the wafer mayresult in yield loss not only from improper earlier processes, but alsofrom problems which can occur due to errors in testing operations. Forexample, during a probe testing operation electrical contact must bemade to the bonding pads of each integrated circuit in order tostimulate electrically the circuit and to measure critical parameters.An array of fine wire probes, conductive bumps and/or fine beams formedon a card is aligned so as to correspond with the array of bonding padsand is used to contact mechanically and electrically the array ofbonding pads. Typically, each die on the wafer is sequentiallypositioned and aligned under the array of probes, for example, and thewafer is moved up to allow contact of the respective probes onto thechip. Precision wafer movement stages allow each chip to be positionedunder the probe array, brought into contact with the probe array, andtested. The chips on the wafer which do not pass the electrical test aremarked by some method such as by applying a dot of ink or by storingtheir respective position on the wafer in computer memory for laterrecall.

In most cases, the interconnecting metal layer or layers of theintegrated circuit chip are formed of aluminum or sometimes gold. Thesemetals provide good processing characteristics and good electricalcharacteristics. However, these metals are also rather soft incomparison with the typical materials used for forming the probes on thecard (referred to herein as an integrated circuit probe card). As aresult, it is likely that damage to the bonding pad area will occur ifthe probe card is not properly constructed, aligned, adjusted andutilized. For example, the tips of the probes must be carefully adjustedfor planarity to insure that all probes touch the respective bondingpads at relatively the same time. The probes must also be adjusted tocontact, e.g., touch down, accurately on each pad. After the probesinitially contact the respective bonding pads, a proper amount ofoverdrive must be maintained past the point of initial contact in orderto provide a contacting force resulting in a consistent low resistancecontact. The tips of the probes themselves must be capable of providinglow resistance contact between the probe and the bonding pads and shouldbe free of contaminants which prevent good electrical contact. Thecontacting force or spring constant of the probe itself is also aparameter which must be considered in determining the ability of a probeto provide a proper contact.

Various technologies have been used to produce probe cards for testingintegrated circuits. The most common types are blade, epoxy ring andmembrane technologies. A fourth type, which involves what is referred toa "buckling beam", also has been used by some manufacturers. Bladetechnology is discussed in U.S. Pat. No. 4,161,692 for a "Probe Devicefor Integrated Circuit Wafers"; U.S. Pat. No. 3,849,728 for a "FixedPoint Probe Card and an Assembly and Repair Fixture Therefor"; and U.S.Pat. No. 4,382,228 for a "Probes for Fixed Point Probe Cards". Epoxyring technology is discussed in U.S. Pat. No. 3,835,381 for a "ProbeCard Including a Multiplicity of Probe Contacts and Methods of Making",U.S. Pat. No. 3,905,008 for a "Microelectronic Test Probe Card Includinga Multiplicity of Probe Contacts and Method of Making Same"; U.S. Pat.No. 4,599,559 for "Test Probe Assembly for IC Chips"; and U.S. Pat. No.4,757,256 for a "High Density Probe Card". Buckling beam technology isdiscussed in U.S. Pat. No. 4,554,506 for a "Modular Test Probe"; andU.S. Pat. No. 4,843,315 for a "Contact Probe Arrangement forElectrically Connecting a Test System to the Contact Pads of a Device tobe Tested".

The most commonly used type of technology to produce integrated circuitprobe cards is epoxy ring technology, although the other technologiesare similar. In the construction of an epoxy ring type probe card, asheet of mylar is punched or drilled with a series of holes in the samearray pattern as the bonding pad locations on the chip. The holes aresized to accept the tip of each probe and hold the tip in positionduring construction of the card. These holes are typically 0.001 inch to0.002 inch in diameter. Each probe is made from a length of spring wirewhich is tapered to a point at one end and bent down at a steep angle toform a probe tip. Each probe tip is placed in a corresponding hole inthe mylar sheet. The other end of each spring wire probe is arrayed in agenerally circular pattern with those of the other probes and is securedin place by a ring of epoxy or another suitable material. The endsprotrude through the epoxy in order to be soldered to a circuit boardwhich forms the probe card. After the probes are soldered to the circuitboard, the probe tips are sanded to provide relatively flat probe tipspositioned in a relatively planar array.

Various systems have been used to inspect the probe cards and, forexample, to adjust the probe cards. The critical parameters for probecards include planarity of the probe tips, contact resistance,electrical leakage from probe to probe and alignment of the probe tipsrelative to the bonding pads. Other important parameters are probe tipdiameter, contact force or spring constant of the probe, and the lengthof the probe tip. Failure of the probe card to meet the requiredparameters can result in errors when using the probe card to evaluatechips on a wafer.

Methods for measuring planarity, contact resistance and leakage areknown in the art. Alignment of the probe tips has typically beenevaluated by way of a visual comparison of the probes to the bonding padarray of the integrated circuit, and more recently, by way of anelectrical method described in U.S. Pat. No. 4,919,374 entitled "Methodand Apparatus for Inspecting Integrated Circuit Probe Cards" and in U.S.Pat. No. 5,060,371 entitled "Method of Making Probe Cards". Themeasurement of probe tip diameter, contact force and the length of theprobe tips has typically been performed using known manual methods.

Determining the planarity of the array of probe tips is typicallyaccomplished using a flat metal plate held parallel to the surface ofthe probe card. The probes are then sequentially scanned to determinewhen contact occurs with the metal plate as the plate is incrementallymoved along a Z-axis so as to be brought into contact with the probes.As each probe makes contact, the position of the plate with respect tothe surface of the probe card is recorded. Thus, the Z-axis positions ofall the probe tips are determined. However, conventional probe cardinspection systems offer very little in the way of assistance to theoperator for adjusting the heights of the probes to form a more planararray. Moreover, if two or more probes in an array are parallelconnected or "bussed" to allow extended current carrying capacity,additional tests must be performed to determine their individual Z-axispositions. A method of isolating individual probes is required todetermine when each probe touches the conducting surface. A variety ofpins, insulators with holes and conducting dots with surroundinginsulator material have been used for this purpose.

Contact resistance of the probe tips can be measured using conventionaltechniques for the measurement of low resistances. A typical methodwould be to bring the probe tip into contact with a conducting metalsurface and measure the resistance of the resulting interface. Afterprobes are in contact with the contacting metal surface, current isforced, via the multiplexer interface, on each probe and then thesubsequent voltage of each probe is measured. The measurement preferablyis performed at a known deflection or "overdrive" of the probe after theinitial contact with the surface. Since the resistances are typicallyvery small, e.g., on the order of a few tenths of an Ohm, Kelvinmeasurement techniques are required for accurate measurements. The typeof metal used for the contact plate is typically gold and somedifferences will be observed between the resistance measured by theseconventional methods and the actual resistance observed when the probeis contacting bonding pads formed using aluminum metalization, forexample, on the integrated circuit chip.

Furthermore, since the aluminum is rather soft in comparison with theprobe tip material, the tip of the probe will tend to protrude or "dig"into the aluminum and make contact over a much larger surface area ofthe tip as compared to on the harder gold surface. The angle of theprobe tip relative to the bonding pad is such that a scrubbing motion iscreated when the tip is driven against the pad. In the case of a bondingpad made of soft aluminum, this creates a scrub mark corresponding tothe path of the probe tip on the pad. Accordingly, it will beappreciated that the more that is known about the size and shape of theprobe tips and the scrub marks they create, the more complete theunderstanding of the ability of the probe tip to maintain low contactresistance.

Electrical leakage between probes on the probe card may adversely affectthe testing of the integrated circuit. Therefore, it is important toinspect for electrical leakage. Leakage may be measured by applying apotential to each probe in sequence and grounding the remaining otherprobes, and then measuring the current which flows. However, the testmust take into account any bussed probes or components such asresistors, diodes and capacitors which may be permanently connected tothe probes and form a part of the probe card. In order to perform acomplete inspection of the probe card for electrical leakage, it isnecessary to check each probe relative to every other probe to ensurethere are no leakage paths.

Conventional inspection systems do not distinguish which tests in aseries of different tests should be performed first on the probe card.However, in order to properly test for planarity it is desirable to testfirst for leakage to avoid incorrect results from probes which haveexcessive leakage.

The X,Y locations of the probes (corresponding to an X-axis and Y-axis)with respect to the bonding pad locations on the integrated circuit havetypically been adjusted using manual means and comparing the X,Ylocations to an actual integrated circuit chip or a film representationthereof. The probes, or the scrub marks on the pads, provide the visualcomparison for the operator to determine which direction to bend orreposition the probes in order to achieve proper alignment. Thisalignment task becomes increasingly difficult and prone to operatorerror with high numbers of probes in a given array. The electricalmethod disclosed in U.S. Pat. Nos. 4,919,374 and 5,060,371 determinesthe X,Y positions of the probes by determining the contact point of eachprobe on orthogonal conducting strips on an insulating material. Thistechnique is only suitable for coarse approximations, however, as theflat shape of each probe tip causes actual the contact point between theprobe tip and the conducting strips to be offset from the center of theprobe tip. This results in measurement errors which are proportional tothe diameter of the probe or the distance thereacross if shaped otherthan a circle. Also, the technique requires that the probes be broughtinto contact with the insulating material and conducting strips a verylarge number of times in order to determine the X,Y positions of all theprobes. This can result in excessive wear on the probe tips. Forexample, because the insulating material is typically ceramic, the probetips may be abraded excessively and pick up contaminants from the roughceramic surface.

In view of the aforementioned shortcomings associated with conventionalprobe card inspection systems, it is desirable to have an inspectionsystem which tests the X,Y locations of the probes on the probe cardwithout requiring an actual wafer or a film representation of thebonding pads. The design information of the pad locations and sizes istypically available in computer readable form before the photomasks orthe wafers are produced. It is desirable to import this informationdirectly into the inspection system performing the X,Y location test toavoid errors and to be able to produce a probe card even before wafersare available. It is also desirable that the inspection system be ableto determine automatically the proper probe locations and configurationsfrom the photomask used to create pad openings in the protective glasslayer over the chip. In addition, it is desirable that the inspectionsystem be able to use a known good probe card for the same purpose whenother means are not available.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an integratedcircuit probe card inspection system which provides for the automaticinspection of all critical parameters of integrated circuit probe cards.It is a further object of this invention to provide such a systemwhereby automatic inspection of contributory parameters of integratedcircuit probe cards is also provided. It is still another object of thisinvention to enable the inspecting of probe card parameters even beforewafers or art work are available for the specific integrated circuitwhich is to be tested using the probe card. It is still another objectof this invention to provide a system which can determine the properprobe locations and configuration for a probe card from an existingprobe card or pad layer photomask.

Furthermore, it is an object of this invention to provide a system formeasuring the X,Y coordinate locations of the probe points of anintegrated circuit probe card and for comparing the measured locationsto the desired locations. Another object of this invention is to providean inspection system to aid in correcting improperly positioned probetips. Still another object of this invention is to provide a system fordetermining the scrub pattern which will be generated by a probe tipwhen it is positioned onto an integrated circuit. It is still anotherobject of this invention to provide a system for accurately andautomatically determining the shape and size of the probe tips.Moreover, it is an object of this invention to provide a system foraccurately and automatically determining the length of the probe tip.Even further, it is an object of this invention to provide a system forchecking the results of such analysis and adjustments by positioning theprobes on an actual integrated circuit chip on a wafer.

These and other objects of the invention are provided by an inspectionsystem according to the present invention for inspecting and adjustingthe probes on probe cards of various types, including but not limited toblade, epoxy ring, membrane and buckling beam type probe cards.

According to one particular aspect of the present invention, anintegrated circuit probe card inspection system is provided whichincludes an integrated circuit probe card inspection system fordetermining the relative location of probes in a probe array, comprisinga viewing system providing an image of the tip of each probe in digitalform, a window with a flat surface contacted by said probe tip, acomputer means with software means to analyze the probe image positionwithin the video image, and positioning means to position the center ofthe digital image to a known physical position with said probe contactin the field of view.

According to another aspect of the present invention, an integratedcircuit probe card inspection system is provided comprising anintegrated circuit probe card inspection system for determining thelocation and length of the scrub mark which would be made by a probe tipon an integrated circuit bonding pad, comprising a viewing systemproviding an image of the probe tip in digital form, a window with aflat surface contacted by said probe tip, a computer means with softwaremeans to analyze the probe tip image position within the video image,and positioning means to position the center of the digital image to aknown physical position with said probe tip in the field of view.

According to still another aspect of the present invention, anintegrated circuit probe card inspection system is provided comprising amethod for determining the location and length of the scrub mark made bya probe on an integrated circuit bonding pad contacted by the probe,said method comprising the steps of capturing a first digitized image ata first defined overdrive, and capturing a second digitized image at asecond defined overdrive, then determining the path and form of thescrub mark from the position and size of the two digitized images.

According to still another aspect of the present invention, anintegrated circuit probe card inspection system is provided comprisingan apparatus for determining the length of a probe tip, comprisingcontacting means for sequentially contacting the tip and the beam of aspring contact probe, positioning means for controlling X,Y and Z axismovements of said contacting means, measuring means for determining theangle of the scrub mark created by the probe tip upon contact andoverdrive against a surface, measuring means for determining theposition of the beam portion of the probe from the position of the tipand the angle of the scrub mark, measuring means for determining thevertical height of each contacted point from a known reference, andcalculating means for determining the difference in the two measuredheights representing the length of the probe tip.

According to still another aspect of the present invention, anintegrated circuit probe card inspection system is provided comprising amethod for learning the probe tip locations of an existing known goodprobe card by capturing a digitized image of each probe tip on the probecard, then determining the relative position of each probe tip withrespect to the other probe tips on the probe card, then constructing afile of said relative position information for use in determining thecorrect placement of probe tips on other probe cards of the same type.

According to still another aspect of the present invention, anintegrated circuit probe card inspection system is provided comprising amethod for determining the orientation and spatial position of an arrayof probes with respect the test apparatus for determining probe positionby positioning the video microscope field of view within the array ofprobes, and moving in a known direction along the X or Y axis a distancenot exceeding the X or Y dimension of the chip corresponding to theprobe card, and checking for probes in the field of view, and if noprobes are found along the original axis selected, moving along theopposite direction of that axis and along the positive and negativedirections of the other axis, no more than the dimension of the chip inthat axis, until probes are found, and digitizing the image of any probetips found by the video microscope, and determining by electrical meanswhich probe of the array is being viewed by the video microscope, andcomparing the information thus obtained to the X,Y probe locations ofthe probe array to determine the orientation of the probe array withrespect to the X and Y axes and the location of at least one probe inthe array,

According to still another aspect of the present invention, anintegrated circuit probe card inspection system is provided comprising amethod for sanding probe tips to a known extent by determining thevertical height of at least the lowest probe of an array of probes bymaking contact with a planar metallic surface, and moving the array ofprobes over a metallic surface sanding area which is co-planar with saidplanar metallic surface, and moving said metallic surface sanding areauntil electrical contact is made with the lowest probe in the array, andfurther moving said metallic surface sanding area into physical andelectrical contact with said array of probes to a known amount desiredto provide proper sanding.

According to still another aspect of the present invention, anintegrated circuit probe card inspection system is provided comprisingmeans for determining the relative locations of bonding pads of anintegrated circuit chip, comprising, in combination a photomask of thepassivation layer of said integrated circuit, a viewing system providingan image of each bonding pad area of said photomask in digital form,holding means to position said photomask in a known orientation to theviewing system, lighting means to enhance contrast between clear anddark areas of said photomask, computer means with software means toanalyze said bonding pad image position within the video image, andpositioning means to position the center of the digital image to a knownphysical position with each of said bonding pads sequentially in thefield of view.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrativeembodiments of the invention. These embodiments are indicative, however,of but a few of the various ways in which the principles of theinvention may be employed. Other objects, advantages and novel featuresof the invention will become apparent from the following detaileddescription of the invention when considered in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a typical semiconductor waferincluding an exploded view of semiconductor chip on the wafer showingthe bonding pad areas.

FIG. 2 is a cross-sectional view of a typical integrated circuit probecard.

FIG. 3 is a top view of a combination wafer chuck and testing surfaceaccording to a preferred embodiment of the present invention.

FIG. 4 is a cross-sectional view of the wafer chuck and the associatedCCD camera and lensing apparatus according to the present invention.

FIG. 5 is a cross-sectional view of a portion of the wafer chuckincluding the bus probe pin according to the present invention.

FIGS. 6(a)-6(c) are schematic views of video images of probe tips atzero overdrive and at the specified overdrive, and a combined viewrepresenting the scrub mark created by the probe, respectively, inaccordance with the present invention.

FIGS. 7(a) and 7(b) are cross-sectional views of a portion of the waferchuck including the bus probe pin shown contacting the tip andsubsequently the beam or shank of a probe, respectively, according tothe present invention.

FIG. 8 is a block diagram representation of the computer and electronicsused to control the movable stage, video system and measurement systemaccording to the present invention.

FIG. 9 is a representation of a movable stage assembly including a waferchuck according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described with reference to thedrawings, wherein like reference numerals are used to refer to likeelements throughout.

Referring initially to FIG. 1, a typical semiconductor wafer 10 is shownincluding a plurality of integrated circuit chips 20. An enlarged viewof one of the chips 20 is provided so as to illustrate a plurality ofbonding pads 40. In order to determine which chips 20 on the wafer 10meet the desired specifications, electrical tests are performed on eachchip 20 using an integrated circuit probe card (FIG. 2). Electricalcontact to the respective chips 20 is made via the respective bondingpads 40. These bonding pads 40 serve the dual purpose of providingconnection points for bonding wires (not shown) which provide conductingpaths from the chip 20 to its package or carrier, and providing contactpoints for probes to provide connection to a suitable probe card forelectrically testing the chips 20 on the wafer 10.

A typical probe card 45 for testing integrated circuit chips 20 in waferform is shown in FIG. 2. The probe card 45 includes a printed circuitboard 50 to which are attached a plurality of fine wire probes 60. Theseprobes 60 are typically attached to traces on the printed circuit board50 by solder connections 70. The tips of the probes 60 are designated 90and are arrayed in a pattern to match the bonding pad configuration ofthe chip 20 to be tested. The probes 60 are held in their particularconfiguration by an epoxy or ceramic ring 80 as mentioned above. Thefine wire probes 60 are typically formed of tungsten, beryllium copperor another suitable metal. The tips 90 of the probes 60 are typicallyfrom 0.0005 inch to 0.003 inch in diameter. The tips 90 must becarefully positioned to contact the bonding pads 40 (FIG. 1) in such away as to provide reliable contact to the respective pads 40. Theconfiguration of the probes 60 is such that as the probe tips 90 arepressed against the bonding pads 40, some lateral motion of the tipacross the pad 40 called "scrubbing" occurs. If all of the probe tips 90were in exactly the same horizontal plane parallel to the printedcircuit board 50, this scrubbing contact action on the pads 40 would bevery predictable and repeatable. Because of the physical processesrequired to construct the probe card 45, however, the probe tips 90 willnot be perfectly planar with respect to the surface of the printedcircuit board 50 or the surface of the chip 20 on which the bonding padsare located. If the probe tips 90 are sufficiently out of planarization(i.e., out of a planar state), the scrubbing action of the lowerpositioned probe tips 90 may cause those probe tips 90 to travel off ofthe respective pads 40 before the higher positioned probe tips 90 makecontact with the pads 40. Similarly, the probe tips 90 will not beperfectly aligned to make contact in exactly the same spot on all pads40. Misalignment of the probe tips 90 with the pads can cause the tip tomiss the pad 40 and/or possibly damage the protective oxide layer whichsurrounds the pad 40. As a result, it is necessary to planarize (i.e.,make planar) the probe tips 90 and adjust their position to properlycontact the pads 40 in order to assure reliable contact for testing.

Referring now to FIGS. 4 and 8, the system of the present invention,designated 99, includes a movable stage 93 which provides accurate,computer controlled motion in the X,Y,Z and theta (θ) directions. Awafer chuck 100 is mounted to the stage 93 and includes a planarizingsurface 110 for determining the planarity of the probe tips 60, aviewing window 120 flat and parallel to the planarizing surface 110which enables a video camera to view the probe tips, a bus pin probe 130for isolating common or bussed probes, and an area including a sandingmedia 140 for cleaning probe tips 90.

The system 99 includes a video camera 124 to view the probe tips 90 frombeneath the probe card 45 to determine X,Y alignment. The probe card 45is fixedly positioned relative to the stage 93 above wafer chuck 100.The camera 124 moves with the X,Y and Z motion of the stage 93 and ispositioned so that the viewing port of the camera 124 is in the centerof the wafer chuck 100. A lens system 126 with coaxial lighting isconnected to the camera 124 and the combination is mounted horizontallyunder the wafer chuck 100. A right angle prism 127 directs thehorizontal view of the camera 124 through the lens system 126 up throughthe viewing window 120 at the center of the wafer chuck 100. The viewingwindow 120 in the surface of the wafer chuck 100 provides a surface 122for exerting a driving force on the probe tips 90 under observation tothe desired level of overdrive for viewing by the video system. As thestage 93 raises the surface 122 incrementally in the Z-direction towardsthe probe tips 90, the surface 122 presses against the probe tips toexert the driving force. The video camera 124 is focused just above thetop surface 122 of the window 120 where the probe tip under test islocated. A combination of top and bottom lighting of the probes 60 ispreferred to view the probe tips 90 to determine X,Y position, size,shape and condition.

Video images obtained by the video camera 124 are captured in a computerusing a video processing board which stores digital representations ofthe video images of the probe tips. Depending on the density (pitch) ofthe probe card 45, one or more probe points are captured in each image.After each image is captured, it is analyzed using image analysistechniques to determine the position of the centroid of each probe tip90 in the image. Since the position of the pixels of the image withrespect to the position of the stage is known (from system calibration),the X,Y position of each probe tip 90 may be determined. The image ofeach probe tip 90 in the array is preferably captured and its positiondetermined in this manner. It is particularly desirable to capture theimage of the probe in the desired overdrive condition. This overdrivecondition creates special problems with regard to common or bussedprobes, and the inspection system 99 provides a means for analyzingbussed probes as is discussed in detail below.

As is described in more detail below with respect to FIG. 3, a surfaceportion 110 of the wafer chuck 100 is smooth and flat with gold platingthereon to provide a planarization area which is electricallyconductive. A bus probe pin 130 is extended through the wafer chuck 100outside the planarization area 110 for use when analyzing bussed probes.The pin 130 may be retracted below the surface of the chuck 100 to allowa wafer to be placed on the chuck 100. In the center of the chuck 100 isthe viewing window 120 for the video camera 124 which allows a computerto capture images of the probe tips. These images in combination withthe X,Y,Z positions of the stage 91 provide the information necessary todetermine the position of each of the probe tips. The viewing windowsurface 122 is in the same plane as the surface of the planarizationarea 110 of the chuck so that a known deflection can be applied to theprobes as they are driven against the viewing window surface 122. Thewindow surface 122 is constructed of a hard, transparent material suchas sapphire which will not be scratched or damaged by the probe tips.

Measurement circuitry and multiplexers allow each probe in the array tobe electrically stimulated and measured under control of the computer asis described below in more detail with respect to FIG. 8. A singlecomputer may be used to provide control of the stage assembly,measurement circuitry and multiplexers as well as providing a userinterface to the operator. However, it will be appreciated that multiplecomputers can be provided to carry out the same tasks.

The sequence of analysis is also important to the proper analysis of aprobe card 45. The electrical leakage test preferably is performed firstto eliminate the possibility of low resistance paths effectivelyshorting probes together. Otherwise, if the planarization test wereattempted with two or more probes unintentionally shorted together, anyone of the shorted probes touching the chuck surface 110 would berecorded as a touch for all of the group of shorted probes and false Zvalues would be recorded for the others.

Referring again to FIG. 4, the planarization test preferably isperformed next in the sequence in order to know the vertical or planarposition in the Z-direction of each probe with respect to the chuck 100.This data is then used to deflect each probe to the proper overdrivecondition in order to find each probe's X,Y position in the overdrivecondition. As an example, the lowest positioned probe in the array(relative to the Z-axis), i.e., the probe to first contact theplanarization area as the chuck surface 100 approaches the probe card45, is found and overdrive values determined from its position. Toperform the planarization test a ground potential is placed on the chucksurface 110. The probe card 45 is fixtured in a holder to be above andparallel to the chuck with the probe tips facing downward toward thechuck. Multiplexed measurement circuitry described with respect to FIG.8 provides access to each probe 60 for electrical stimulus andmeasurement. The chuck 100 is incrementally raised via the stage 93 andthe array of probes is scanned with a positive voltage at each step todetermine when each probe 90 touches the chuck surface 110, thus makingcontact to ground and causing a current flow which is detected. TheZ-axis position of the stage 91 is recorded for each touch of a probe60, thus defining the Z height of each probe 60. This Z height can beeasily referenced to the surface of the probe card 45 by calibrating theZ position of the stage to the holding fixture (not shown) for the probecard. A "best fit planar line" is established at the Z heightrepresenting the maximum density of probes, such that for a givenplanarization specification, the fewest number of probes 60 would haveto be adjusted to fall within the planar window. The planar window isdefined as limit values equally spaced around the planar line.

If there are bus connected probes 60 on the probe card 45, only theposition of the lowest probe of any bussed group is recorded by theaforementioned planarization test. In order to determine the individualheights of the bussed probes, they must be individually contacted. Thesmall diameter pin 130, raised above the surface of the chuck 100 ispositioned beneath the bussed probes and is used to contact selectivelya single pin at a time in the group. In order to perform this test,however, it is necessary to know the X,Y positions of the probes 60 inthe bussed group. Therefore, the next step in the inspection procedureis to determine the position of each non-affected probe by the videosystem described above. Each independent probe, i.e., each probe that isnot bus connected, may be driven to a specified overdrive as itsvertical position is known exactly. The probes which are bus connected,or which have resistor or capacitance connections, may also be located,but the exact overdrive position is unknown at this point. After the X,Yposition of these probes is known, however, they may be individuallytested using the bus probe pin 130 as described below to determine theirexact planar position, then retested for exact X,Y position at theproper overdrive position.

Since the array of probes 60 may be presented to the present system inany orientation relative to the θ position of the chuck 100, and thevideo camera 124 and the related optical means have no way ofdistinguishing which probe 60 is being tested, a method according to thepresent invention is used to identify orientation of the array and eachindividual probe. If it is assumed that the viewing window 120 is placedanywhere within the array of probes at the start of the test, i.e, belowat least one probe 60, the system 99 can automatically determine therequired information. "Stepping" or movement of the stage 91 in anydirection, up to a distance equal to the array size, must encounterprobes 60. When a probe or probes is found via the video camera 124, thesystem 99 records the position or positions. At this point, it is stillnot known specifically which probe or probes have been found. However,because the position of a probe or probes known to be in the array isnow known, the identity of any or all of the probes may be determined bycontact with the bus probe 130 to identify the probe by its electricalconnection in the matrix. If a bus or common probe is selected, theprocedure must be continued until an individual probe is found. Theknown position of one individual probe plus the specified position dataof the probes in the array is sufficient to determine the orientationand expected positions of all the probes.

When the system 99 has established absolute X,Y position data for allprobes 60 in the probe card 45, the next step is for the system toevaluate the relative positions of each probe to all others. A simpleapproach is to use one probe as a reference (such as a probe pin 1) andto determine proper relative position for all other probes withreference to that probe. However, this can result in seriouscomplications if the reference probe itself is improperly positioned. Inorder to assure that the minimum number of probes 60 that need beadjusted to bring the relative positions of the entire array of probesinto specification, a best fit analysis is made on the array of bondingpads 40 (FIG. 1) to be contacted. This is done by the computer usingmatrix techniques which mathematically move the array of measured X,Yprobe positions around to match as closely as possible the X,Y padlocations (which are known form the chip 20 design) for best fit. It isthen determined which probes are not properly aligned to the pads, perthe specification, and those probes are selected for further analysisand adjustment.

By finding the locations of the probe tips at zero overdrive and at aspecified nonzero overdrive, the difference in the two locations can becalculated to determine the path and length of the scrub mark created bythe probe as is discussed below with respect to FIGS. 6(a)-6(c). Theangle of this scrub mark may also then be used to determine the angle ofthe probe beam with respect to the X or Y axis.

In order to determine the length of the probe tip, the difference inheight of the tip 90 of the probe and the beam of the probe 60immediately prior to the bend must be found. The bus probe pin 130 maybe used to find these two heights by alternately positioning it underthe tip of the probe, finding the Z height, then positioning it underthe beam, finding the Z height, and then calculating the differencebetween the two heights as is described below with respect to FIGS. 7(a)and 7(b). The proper X,Y location for the second measurement is found byusing the angle of the scrub mark as described above.

Referring briefly to FIG. 9, the movable stage assembly 93 is shown indetail. The chuck 100 is mounted to the standard X,Y,Z stage assembly93. As is shown, the Y-axis movement section 95 is on the bottom,extending away from the viewer as seen in FIG. 9. The X-axis movementsection 94, which moves left and right with respect to the drawing, ismounted on top of the Y-axis movement section 95. The Z-axis movementsection 96 moves up and down relative to the drawing and is mounted ontop of the X-axis movement section 94. The three axes of motion X,Y,Zmay be driven with standard stepper or servo motors 97.

Referring now to FIG. 8, the stage 93 is controlled by a computer 200via a motor controller card 230 and motor drivers 240 which drive eachof the motors 97, respectively. The arrangement of the computer 200 issuch that software control of the stage 93 allows accurately positioningof the chuck 100 in the X,Y,Z directions as required for testing theprobe card 45. Known programming techniques such as those used withcomputerized milling machines can be used to provide the appropriatecomputer control as will be appreciated. As a result, detail on thecomputer software is omitted.

The computer 200 is interfaced with special peripheral driver cards asis shown. These special peripheral driver cards include a video imagecapture card 250, an analog measurement card 210, and the aforementionedmotor controller card 230. The video image capture card 250 interfacesthe video camera 124 to the computer 200 to provide digitalrepresentations of the video images obtained by the video camera 124.The video camera 124 can be, for example, a charge-coupled device (CCD)camera. The analog measurement card 210 provides electrical stimuli andmeasurement capabilities to force and to measure voltages andresistances relative to the probes 60. Multiplexer cards 220 eachselectively provide a connection path between the analog measurementcard 210 and each probe 60 of the probe card 50 under the control of thecomputer 200 via the analog card 210. The analog measurement card 210 isalso connected to the chuck 100, and particularly to the conductivesurface 110 and the bus probe pin 130 to complete the electricalmeasurement path for planarity and contact resistance measurements. Themotor controller card 230, as is mentioned above, provides an interfaceto control the stepper or servo motors 97 driving the X,Y and Z axes ofthe stage 93 FIG. 9). Each motor is interfaced to the motor controllercard 230 via standard motor drivers 240 specific to the type motor whichis used as is conventional.

Referring to FIG. 3, the chuck 100 is shown in detail. As mentionedabove, the chuck 100 includes a flat parallel surface 110 which iselectrically conductive and is provided to allow testing the planarityof the probe tips 90. A viewing window 120 in the center of the chuck100 provides for viewing the probe tips 90 with the video camera for thepurpose of determining the X,Y coordinate locations of the probe tips90. The bus probe pin 130 is provided to allow isolating individualprobe tips 90 which are connected together (i.e., bussed) in a commonelectrical network. The pin 130 is also used to measure the verticalforce or spring constant of each probe 60 as it is driven against asurface. This measurement provides an indication of the amount of forceapplied by the probe tip 90 to the pad 40 of the integrated circuit chip20 during the wafer probe operation. In addition, a small area sandingplate 140 is provided on the chuck 100 for the purpose of removingcontamination from the probe tips 90 during testing so as to providebetter contact surfaces on the probe tips 90 and brighter tips 90 forviewing by the video system. The chuck 100 is also equipped with vacuumport grooves 150 to hold down a wafer 10 on the chuck 100. This featuremay be used to confirm the results of any testing and adjustments byallowing the user to contact the probe tips 90 to the bonding pads 40and observe the scrub marks formed in the relatively soft padmetalization for proper X,Y positioning and length of the scrub mark.

If the probe card 45 is positioned over the flat surface 110 of thechuck 100 such that the printed circuit board 50 is parallel to thesurface 110 as is shown in FIG. 4, a conducting plane formed by the flatsurface 110 exists to determine the relative planar positions of theprobe tips 90. The chuck 100 and consequently the flat surface 110 isincrementally moved via the movable stage 93 into contact with the probetips 90. For clarity, a portion of the movable stage 93 has been omittedfrom FIG. 4. At each increment, the probe tips 90 are individuallychecked electrically for contact with the flat surface 110 via theanalog measurement card 210 and the multiplexer cards 230. As contact isfound with any probe tip 90 (noted by the detection of current flow, forexample), the vertical position of the chuck 100 (relative to theZ-axis) is recorded by the computer 200. When all probe tips 90 havebeen located in this manner, or the maximum desired overdrive positionbeyond the first contact found is achieved, the process is terminatedand the relative vertical height data of the probe tips 90 with respectto each other is obtained. This information may then be used todetermine how much and in which direction the individual probes 60should be adjusted by bending them up or down in order to be uniform inheight or planar position. The Z-axis movement section 96 (FIG. 9) mustbe sufficiently accurate and have fine enough step resolution to map thevertical positions of the probe tips 90 to the required degree ofaccuracy. For present technology, this resolution and accuracy is on theorder of 1.0 to 3.0 micrometers. The probe tips 90 are typicallyadjusted to be coplanar within 10 to 25 micrometers.

When the probes 60 are connected to a common node, or bus connected, themethod described above will only find the lowest positioned probe tip 90in the bus connected group. Each probe tip 90 in the group must then beelectrically isolated from others in the group in order to find itsheight. This is accomplished by using the bus probe pin 130 in itsraised position as is shown in FIG. 5 to isolate a probe tip 90 and toindividually contact it to the chuck 100. The height of the bus probepin 130 (relative to the Z-axis) may be calibrated to be a known heightabove the surface 110 in its raised position, thus providing a knownheight for the measured probe tip 90 relative to all other probe tips90. FIG. 5 shows the relative position of the bus probe pin 130 withrespect to a portion of the wafer chuck 100. Because the bus probe pin130 is electrically connected to the analog measurement card 210, thepin 130 can be held at a ground potential in the same manner as thesurface 100. Thus, when the bus probe pin 130 contacts a particularprobe pin 60 selected by the multiplexers 220 and having a positivepotential applied thereto, such contact is identified by the detectionof current flow by the analog measurement card 210.

The bus probe pin 130 also has another very important use in the presentinvention. Referring to FIG. 4, a cross section of the chuck 100 isshown including the video window 120 area. The video window 120 includesa hole 121 through the chuck 100 and a sapphire window 122 within thehole 121 which is coplanar with the surface 110 of the wafer chuck 100.The video system 146 which is integral with the chuck 100 and moves withit via the stage 93, includes the video camera 124 coupled to an opticalmicroscope 125 with a small diameter lens system 126 and focusingmechanism 130'. The lens system 126 includes a fiber optic light guide126' (shown in phantom) which is coaxially arrayed around the outside ofthe lens system 126 to provide sufficient light directed onto the probetips 90 via a right angle prism 127. The image of the tip of a probe 60or probes directly above the window 120 is directed at right angles tothe lens system 126 by the right angle prism 127. A holding mechanism128 allows the system to be adjusted so that the view of an objectthrough the window 120, in the illustrated case the tip of a probe 60,is centered relative to the lens system 126. The focus mechanism 130'allows the optical microscope 125 to be focused accurately on thesurface of the sapphire window 122, hence on the tip 90 of the probe 60.The video output of the video camera 124 is provided to a commerciallyavailable image capture card 250 such as that available from VolantSystems, Inc. (Model No. 2510) operating in the computer 200. Thecomputer 200 can be a personal computer. Blob analysis software runningon the computer 200 to control the image capture 250 is used todetermine the position of image of the probe tip 90 on the video fieldof view. Since the X,Y position of the chuck 100 and window 120 may bedetermined from the position of the stage 93 which includes stepper orservomotor control and linear position encoders, the position of theprobe tip 90 can be determined. When the X,Y position of a probe tip 90is determined by the video system 146, the bus probe pin 130 is used toidentify which probe tip 90 has been located. This is accomplished bycontacting the located probe tip 90 with the bus probe pin 130 andchecking all possible probe tips 90 for contact with the pin 130 via theanalog measurement card 210 and multiplexers 220 to determine whichprobe tip 90 is being contacted. Once a pattern has been established,the system can identify probe tips 90 strictly by their positions basedon comparison to the known X,Y positions of probe tips 90 in the array.

Referring still to FIG. 4, the sequence in which the various operationsare performed is a particular feature of the subject invention. Thefirst step is to place the probe card 45 in a known area so that theinspection system 99 may locate the array of probes 60 through thewindow 120 with the video system 146. The preferred method is to movethe stage 93 such that the video window 120 is positioned somewheredirectly beneath the array of probes 60. The control software for thecomputer 200 is written to automatically perform the required operationsto determine X,Y, and Z locations of all the probe tips 90 based on theblob analysis software. The inspection system 99 then checks forelectrical leakage or shorts between each probe 60 and all other probes60. This assures that all electrical measurements to determine probeidentities will give accurate results. The next step is to determine thelowest probe tip 90. At this time, the height of all thenon-bus-connected probes 90 may be determined relative to the Z-axis asdescribed above along with their contact resistance as measured betweenthe surface 110 of the chuck 100 and the probe tip 90. This isaccomplished by moving the stage 93 such that the array of probes 60 isover the flat parallel surface 110 of the chuck 100 and offset from thevideo window 120. After this is complete, the video window 120 can beautomatically returned to its position within the array of probes 60. Bysearching alternately in the X or Y direction to a distance notexceeding the X or Y dimension of the chip 30 for which the probe card50 is designed, the video system 146 must encounter probe tips 90 in itsfield of view. Once a probe 90 is found, it must be identified. This isaccomplished as explained above by the use of the bus probe pin 130. Ifthe identified probe 90 in not a bus probe 90, i.e., the probe 90 iselectrically isolated, it may be identified uniquely. If a bus probe 90is found, the procedure must be continued until a known unique probe 90is located. Comparison to information on the array of probes 60 andpositions of the probe tips 90 stored in the computer 200, theorientation of the array relative to he chuck 100 may be determined. Theprocedure can then continue to locate the probe tips 90 with the videosystem and for bus probes 90, determine their individual Z-axis heightsand contact resistances using the bus probe pin 130.

Referring to FIGS. 6(a)-6(c), typical views of a probe tip 90 as seen bythe video system 146 are shown. In the first video image 150 shown inFIG. 6(a) a probe tip 90 is shown as it would initially contact thevideo window 122 without being deflected or overdriven (i.e., the zerooverdrive position). Since the probe 60 X,Y positions are normallyspecified at a known overdrive, the Z-axis height of the chuck 100 ismoved up from the height at which initial contact occurred in order todeflect the probe 60 to the specified overdrive as shown in the secondvideo image 160 illustrated in FIG. 6(b). The system 99 may compare thelocations for each probe 90 as found in the overdriven position of thesecond video image 160, or may calculate the path of the probe 90 as ittraverses over the surface as in the superimposed video image 170represented in FIG. 6(c). The difference in location between zerooverdrive position 171 and the specified overdrive position 172 asidentified by the computer 200 is used by the computer to determine theequivalent of a scrub mark 173 which would be made by the probe tip 90on the relatively soft metal of an integrated circuit bonding pad 40(FIG. 1). The specifications on the array of probes 60 may be written torequire that the position of a given probe tip 90 in the overdriven caseor that the equivalent scrub mark 173 be within specified locationsrelative to the respective bonding pad 40 of the integrated circuit chip20. The computer 200 processes the image data obtained from the videosystem 146 to evaluate compliance with such specifications.

The image of the probe tip 90 as viewed by the video system 146 may alsobe used to determine the diameter D (FIG. 6(a)) of the probe tip 90. Aspreviously mentioned, each wire from which the probes 60 are made istypically tapered at one end to form the probe tip 90. As the end of thetip 90 wears or "dulls", the diameter of the probe tip 90 increases.Specifications may be placed on maximum probe tip 90 diameter todetermine when a probe card 50 should be replaced. Again, the computer200 is used to process the image data as shown in FIG. 6(a) usingconventional techniques to determine if the probe tips 90 meet thedesired specifications.

In FIG. 7(a) there is illustrated the condition where a probe tip 90 isplaced in contact with the bus probe pin 130 via the stage 93 for thepurpose of finding the length of the probe tip 90 in the Z direction. Asis known, as the probe tips 90 are used to contact the metalized bondingpads 40 of the integrated circuits 20, the probe tips 90 wear down fromthe abrasion with the oxides on the metal surface. This shortens thelength of the tip area 91 and eventually renders the probe card 45unusable. A good indication of the wear on the probe tips 90 is ameasurement of the length of the tip section 91. FIG. 7(b) shows the busprobe tip 130 being used to contact the shank area 92 of the probe. Theangle of the scrub mark 173 created by the probe tip 90 may be used todetermine the position of the shank area 92 of the probe 60 with respectto the probe tip 90. The difference in height in the Z direction fromthe probe tip 90 to a position on the shank 92 is used by the computerto determine the length of the probe tip section 91 and can be computerby the computer 200. Specifications on minimum probe tip section 91length may be used to determine when a particular probe card 50 shouldbe replaced.

Although the invention has been shown and described with respect tocertain preferred embodiments, it is obvious that equivalents andmodifications will occur to others skilled in the art upon the readingand understanding of the specification. The present invention includesall such equivalents and modifications, and is limited only by the scopeof the following claims.

What is claimed is:
 1. An integrated circuit probe card inspectionsystem for determining the location and length of a scrub mark whichwould be made by a probe tip on an integrated circuit bonding pad,comprising:a viewing system for providing a digital image of the probetip, a window with a flat surface contacted by said probe tip, saidviewing system obtaining said digital image through said window in afirst state where said probe tip is driven in contact with said windowwith a first force, and in a second state where said probe tip is drivenin contact with said window with a second force, said second force beingdifferent from said first force, a computerized means with softwaremeans to analyze the position of the probe tip within the digital imagein said first and second states, and for determining the location andlength of the scrub mark based on said positions.
 2. An integratedcircuit probe card inspection system according to claim 1, wherein thecomputerized means analyzes the scrub mark to evaluate compliance with apredefined specification.
 3. An integrated circuit probe card inspectionsystem according to claim 1, wherein the first force represents asubstantially zero overdrive condition and the second force represents anon-zero overdrive condition.
 4. A method for determining the locationand length of scrub mark which would be made by a probe on an integratedcircuit bonding pad contacted by the probe, said method comprising thesteps of:capturing a first digitized image at a first defined overdrive,and capturing a second digitized image at a second defined overdrive,then determining the path and form of the scrub mark from the positionand size of the two digitized images.
 5. A method according to claim 4,further comprising the step of analyzing the scrub mark to evaluatecompliance with a predefined specification.
 6. A method according toclaim 4, wherein the first defined overdrive represents a substantiallyzero overdrive condition and the second defined overdrive represents anon-zero overdrive condition.
 7. An integrated circuit probe cardinspection system comprising:a deflection surface; a driving devicewhich drives a probe tip into contact with the deflection surface at afirst force and a second different force; a location-determining devicewhich determines a first location of the probe tip when it is driveninto contact with the deflection surface at the first force and a secondlocation when it is driven into contact with the deflection surface atthe second force; and a processing device which, based on said first andsecond determined locations of the probe tip, predicts the location of ascrub mark that would be made by the probe tip on an integrated circuitbonding pad based on the determined first and second locations of theprobe tip.
 8. An integrated circuit probe card inspection system as setforth in claim 7 wherein the location-determining device comprises aviewing device that obtains a first image of the probe tip when it isdriven into contact with the deflection surface at the first force and asecond image when it is driven into contact with the deflection surfaceat the second force; and wherein the processing device predicts thelocation of the scrub mark based on a comparison of the first and secondimages.
 9. An integrated circuit probe card inspection system as setforth in claim 8 wherein the viewing device comprises a video camerathat obtains video images and a video processing board which storesdigital representations of the video images.
 10. An integrated circuitprobe card inspection system as set forth in claim 9 further comprisinga viewing window for the video camera and wherein the window includesthe deflection surface.
 11. An integrated circuit probe card inspectionsystem as set forth in claim 10 wherein the processing device predictsthe path of the scrub mark.
 12. An integrated circuit probe cardinspection system as set forth in claim 11 wherein the processingdevices predicts the length of the scrub mark.
 13. An integrated circuitprobe card inspection system as set forth in claim 12 wherein theprecessing device predicts the angle of the scrub mark.
 14. Anintegrated circuit probe card inspection system as set forth in claim 11wherein the processing device additionally evaluates compliance of thepredicted scrub mark with a predefined specification.
 15. An integratedcircuit probe card inspection system as set forth in claim 11 whereinthe first force represents a substantially zero overdrive condition andthe second force represents a non-zero overdrive condition.
 16. Anintegrated circuit probe card inspection method comprising the stepsof:predicting the location of a scrub mark which would be made by aprobe tip on an integrated circuit bonding pad contacted by the probetip; and evaluating the predicted location of the scrub mark forcompliance with a predefined specification;wherein said predicting stepcomprises the steps of:driving the probe tip into contact with a surfaceat a first force; driving the probe tip into contact with the surface ata second different force; determining the location of the probe tip whenit is driven into contact with the deflection surface at the firstforce; determining the location of the probe tip when it is driven intocontact with the deflection surface at the second force; and predictingthe location of the scrub mark based on the two determined locations.17. A method as set forth in claim 16 wherein said determining stepscomprise capturing a first image of the probe tip during the firstdriving step and capturing a second image of the probe tip during thesecond driving step and wherein said predicting step includes comparingthe two images to determine the location of the predicted scrub mark.18. A method as set forth in claim 17 wherein said capturing stepsinclude obtaining video images and processing and storing digitalrepresentations of the video images.
 19. A method as set forth in claim18 wherein said step of predicting the location of the scrub markincludes predicting the path of the scrub mark.
 20. A method as setforth in claim 19 wherein said step of predicting the location of thescrub mark includes predicting the length of the scrub mark.
 21. Amethod as set forth in claim 16 wherein said step of predicting thelocation of the scrub mark includes predicting the angle of the scrubmark.
 22. A method as set forth in claim 16 wherein the first drivingstep represents a substantially zero overdrive condition and wherein thesecond driving step represents a non-zero overdrive condition.